This invention relates to an electrochemical apparatus containing a plurality of electrochemical cells, each having an exterior fuel electrode, which can be contacted by a gaseous mixture of fresh feed fuel and recirculated spent fuel, which mixture contains water vapor and can pass through a reforming material prior to contacting the fuel electrodes; where recirculated, hot spent fuel heats the reforming material and the gaseous mixture, and fresh feed fuel and recirculated spent fuel can be mixed, at an ejector exterior to the body of the apparatus, with ejector and fresh feed fuel temperatures not exceeding 400.degree. C.
High temperature, solid oxide electrolyte fuel cell generator apparatus and fuel cell configurations are well known, and taught, for example, in U.S. Pat. Nos. 4,395,468 and 4,490,444, respectively, (Isenberg). There, feed fuel, either H.sub.2 +CO, or previously reformed natural gas, is fed into the apparatus at one end and flows parallel to exterior fuel electrode surfaces of the elongated fuel cells. Spent fuel is combusted with spent oxidant in a separate chamber and then exits the apparatus.
Other high temperature fuel cell generator apparatus designs are known, where spent fuel is recirculated and aspirated into fresh, preheated feed fuel, in the center of the apparatus, at apparent temperatures of approximately 600.degree. C. to 800.degree. C., which mixture is fed through the fuel cells, as taught in U.S. Pat. No. 3,718,506 (Fischer et al.). There, spent fuel exiting from the fuel cells, is mixed with fresh feed fuel, such as propane, at a jet-pump nozzle, for flow from one end to the other of the series-connected fuel cells. This combination of spent fuel with fresh fuel prevents soot formation within the cells. Additional spent fuel mixes with spent air and flows around and through a catalytic afterburner structure surrounding the fuel cells for complete combustion, which heats the fuel cells, allowing efficient operation at approximately 800.degree. C. In U.S. Pat. No. 4,729,931 (Grimble), spent fuel and combusted fuel are mixed, and then drawn into fresh feed fuel at an ejector nozzle, in an exterior chamber of the generator apparatus. The entire mixture then passes through a reforming material, located in the same chamber as the ejector, to form a reformed gas which is fed to contact fuel cells within the apparatus.
Another generator design, to prevent chilling of the fuel cell, is taught in U.S. Pat. No. 4,808,491 (Reichner), where a combusted exhaust of spent fuel and spent oxidant provides corner heating in the generator apparatus. There, fresh feed fuel is fed into a manifold at the bottom of the apparatus, the bottom of which manifold may contain reforming catalyst and may be heated by the combusted exhaust. The feed fuel then flows parallel to the exterior fuel electrode surfaces of the elongated fuel cells. The fresh feed fuel is not mixed with any spent gases within the apparatus.
Natural gas (methane plus ethane, propane, butane and nitrogen) is a likely fuel for many of these fuel cell apparatus. This natural gas must be reformed, that is, converted to carbon monoxide and hydrogen, through the use of a catalyst and excess water vapor, prior to its utilization in the fuel cell. The reforming reaction is endothermic, requiring a supply of heat, and is best performed at temperatures close to 900.degree. C. The heat required for reforming is a significant fraction of the excess heat that results from fuel cell operation.
The use of recirculated spent fuel to provide water vapor and CO.sub.2 for fresh feed fuel, by means of an ejector powered by the inlet fresh fuel pressure, has the potential to result in several problems. The ejector in the typical fuel cell apparatus is located directly in the hot fuel cell environment and is bathed by the hot recirculation gas stream, and the nozzle must be insulated or cooled to prevent carbon deposition from natural gas feed fuel, due to "cracking" which occurs at temperatures above about 400.degree. C.
Also, contaminants, such as silica from the nozzle's thermal insulation, or from other generator insulation, picked up and carried by the spent fuel stream, can cause deactivation of the reformer catalyst. The high temperature, due to ejector and reformer location in the hot recirculation gas stream, also requires a permanently-welded assembly of the ejector into the recirculation ducting, to provide adequate sealing, thus increasing fabrication difficulty and impeding access for maintenance. Thus, carbon deposition, silica transport and high temperature sealing are areas of concern for efficient reformation with recirculation.
What is needed is an apparatus which eliminates carbon deposition concerns at the fresh feed fuel-recirculation gas mixing point, and eliminates the need for high temperature sealing, but which also allows heat exchange to a reformer compartment or material. It is one of the main objects of this invention to provide such an apparatus.